NOx absorber catalysts (NACs) are known, e.g. from U.S. Pat. No. 5,473,887, and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is actively—as opposed to passively—decreased. Such active decrease in oxygen concentration is known as “regeneration” of the NAC's NOx adsorption activity or a “purge” of NOx adsorbed on a NAC. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. gasoline fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, oxygen concentration can be actively adjusted to a desired redox composition intermittently in response to a calculated remaining NOx adsorption capacity of the NAC, e.g. to richer than normal engine running operation (but still lean of stoichiometric), to stoichiometric (i.e. lambda=1 composition) or to rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold.
A typical NAC formulation disclosed in the prior art includes a catalytic oxidation component, such as platinum, a significant quantity, i.e. substantially more than is required for a promoter, of a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:NO+½O2→NO2  (1);andBaO+NO2+½O2→Ba(NO3)2  (2),wherein in reaction (1), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO and/or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+ 3/2O2 or Ba(NO3)2→BaO+2NO2+½O2  (3);andNO+CO→½N2+CO2  (4);(Other reactions include Ba(NO3)2+8H2→BaO+2 NH3+5H2O followed by NH3+NOx→N2+yH2O or 2NH3|2O2+CO→N2+3H2O+CO2 etc.).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide and sequence of catalytic coatings in the exhaust stream.
JP 8-117601 discloses NOx absorber catalyst comprising the complex oxide MO.nAl2O3 (M:alkaline earth metal); wherein the composition ratio n is preferably in the range n=0.8-2.5. In the Examples MO was MgO, wherein n is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 with values of n≧1.5 yielding a pure phase product. The resulting MgO.nAl2O3 product is combined with the alkali metal potassium or lithium, both platinum and rhodium or palladium alone and barium to produce a NOx absorber catalyst.
U.S. Pat. No. 6,350,421 discloses a nitrogen oxide storage material which contains at least one storage component for nitrogen oxides in the form of an oxide, mixed oxide, carbonate or hydroxide of the alkaline earth metals magnesium, calcium, strontium and barium and the alkali metals potassium and caesium on a high surface area support material. The support material can be doped cerium oxide, cerium/zirconium mixed oxide, calcium titanate, strontium titanate etc. The purpose of the cerium oxide dopant is to stabilize the specific surface area of the cerium oxide. Dopants are selected from silicon, scandium etc. In a majority of the examples the dopant is zirconium and the support for the storage component is a cerium/zirconium mixed oxide. A representative example consists of a mixture of three powders: barium on a cerium/zirconium mixed oxide; platinum on an aluminium oxide; and rhodium on an aluminium oxide. Barium on pure cerium oxide is used in a comparison example (see U.S. Pat. No. 6,350,421, Comparison Example 4).
EP 1317953 discloses a NOx absorber catalyst that combines the teachings of JP 8-117601 and U.S. Pat. No. 6,350,421. In particular, the catalyst comprises a homogenous mixed oxide of magnesium oxide and aluminium oxide in a concentration of 1 to 40 wt % based on the total weight of the mixed oxide supporting one or both of platinum and palladium and a nitrogen oxide storage component, such as an oxide, carbonate or hydroxide of magnesium, calcium, strontium or barium, an alkali metal, a rare earth metal or a mixture thereof supported on a metal oxide such as cerium oxide or a cerium mixed oxide, e.g. cerium/zirconium mixed oxide. The use of strontium or barium as nitrogen oxide storage components fixed on a support material of cerium oxide or cerium mixed oxides is said to be especially advantageous. Another partial amount of platinum (see Catalyst C1b) or palladium (see Catalyst C2b) can be deposited directly on the nitrogen oxide storage material. In order to achieve as complete a conversion of the desorbed nitrogen oxides as possible during the regeneration phase, it is said to be advantageous to add rhodium supported on a further support material, preferably optionally stabilised alumina.
WO 2005/092481 discloses a variant of the nitrogen oxide storage material of EP 1317953 which variant comprises a nitrogen oxide storage material which is based on storage compounds of elements selected from the group consisting of magnesium, calcium, strontium, barium, the alkali metals, the rare earth metals and mixtures thereof, wherein a homogeneous magnesium-aluminium mixed oxide doped with cerium oxide is support material for the storage compounds and platinum, wherein the platinum is present either on a different homogeneous magnesium-aluminium mixed oxide doped with cerium oxide from the storage compounds (which is referred to as an oxidation-active component) or the same homogeneous magnesium-aluminium mixed oxide doped with cerium oxide as the storage compounds. Where the platinum is present on the same homogeneous magnesium-aluminium mixed oxide doped with cerium oxide as the storage compounds, the nitrogen oxide storage material includes an oxygen-storing material based on cerium, in particular Ce—Zr mixed oxide. In addition to platinum, palladium can be carried on the oxidation-active component. To achieve very complete conversion of the nitrogen oxides desorbed during regeneration of the storage catalyst rhodium can be carried on a further support material, such as optionally stabilised aluminium oxide. In the specific examples, no platinum group metal is supported on the oxygen-storing material based on cerium, nor is there any suggestion so to do.
WO 2008/067375 discloses NOx storage materials and traps that are said to be resistant to thermal ageing. A nitrogen oxide storage catalyst comprises a coating on a substrate comprising a nitrogen oxide storage material comprising ceria particles having an alkaline earth oxide, such as barium oxide, supported on the particles, the ceria having a crystallite size of between about 10 and 20 nm and the alkaline earth oxide having a crystallite size of between 20 and 40 nm. The coating further comprises a catalytic component, which comprises at least one member of platinum group metals supported on refractory oxide particles, i.e. not on the ceria particles. Refractory oxide particles screened for use in the Examples include aluminas doped with cerium oxide or a mixture of cerium oxide and zirconium oxide, including 90% Al2O3, 10% CeO2; 82% Al2O3, 11% CeO2, 7% ZrO2; and 80% Al2O3, 20% CeO2. However, none of the refractory oxides tested is based on cerium oxide.
U.S. 2002/0053202 discloses an exhaust gas purifying system for a Diesel engine comprising a mixture of a H2 supplying catalyst (Pt/CeO2) and a soluble organic fraction (SOF) adsorbing-oxidising catalyst (Pt/La—SiO2) disposed on a first flow through substrate monolith and NOx absorbing catalyst (Ba/Pt—Rh/Al2O3) disposed on a second flow-through substrate monolith located downstream of the first substrate monolith.
WO 2004/025093 discloses a substrate monolith comprising a NOx absorber, palladium supported on a first support material associated with at least one base metal promoter and platinum supported on a second support material. The base metal promoter can be a reducible oxide, such as an oxide of manganese, iron, copper, tin, cobalt or cerium, and the reducible oxide may be dispersed on the first support material or the support material per se may comprise particulate bulk reducible oxide. The NOx absorber can be at least one alkali metal, at least one alkaline earth metal, at least one rare earth metal e.g. lanthanum or yttrium or any two or more thereof. In a particular embodiment, the NOx absorber includes both Pt and Rh, the latter for catalysing NOx reduction to N2, although the Rh can be disposed downstream of the NOx absorber. In one embodiment, the supported Pt component is in a first layer and the supported Pd component and the associated at least one base metal promoter is in a second layer overlying the first layer. Alternatively, all components can be present in a single washcoat layer. The substrate monolith can be a flow-through substrate monolith or a filter.
Historically, vehicular Diesel engines have been designed to meet some combination of four features: fuel efficiency; control of NOx emissions; power output; and particulate matter control. Early emission standards limited the quantity of carbon monoxide and hydrocarbon that it was permissible to emit, which forced fitment of Diesel oxidation catalysts to meet the standards. By the inception of Euro 5 emission standards were most easily met by tuning the engine to control NOx emissions and providing a filter in the exhaust system to trap particulate matter (the so-called NOx/particulate matter trade-off). For Euro 6 and proposals in the US, it appears to be difficult to meet particulate emission standards (which now include a requirement to reduce particulate number emissions) without a filter, whether additional catalyst-based NOx removal strategies are required in addition. Whilst it is possible, therefore, to conceive of an exhaust system comprising a NOx absorber catalyst on a flow-through substrate monolith in the absence of a filter for use in meeting e.g. European emission standards, generally we expect that a system comprising a NOx absorber catalyst on a flow-through substrate monolith will be used in some combination with a filter, or the NOx absorber catalyst will be coated on a filtering substrate monolith, e.g. a wall-flow filter.
A typical exhaust system arrangement for a light-duty Diesel vehicle comprises a NOx absorber catalyst on a flow-through substrate monolith and a catalysed soot filter (CSF) disposed downstream (i.e. in the ordinary flow direction) thereof. Typical problems associated with NOx absorber catalyst development for use in such systems include NOx storage and NOx regeneration under low temperature, relatively high flow rate and relatively high hydrocarbon exhaust gas conditions. Modern Diesel vehicles generally use an engineering solution known as exhaust gas recirculation (EGR) in order better to control NOx emissions, wherein a portion of the exhaust gas is recirculated to the engine inlet during at least some of an internally programmed engine speed/load map. The point in the exhaust system from where the exhaust gas for EGR is taken contributes to the above problems. One typical arrangement is to take EGR exhaust gas from downstream of the CSF, so-called low pressure (or “long-loop”) EGR.
We have investigated the activity of NOx absorber catalysts comprising a first component of PtPd supported on a homogeneous mixed oxide of magnesium oxide and aluminium oxide (i.e. magnesium aluminate) and a second component of a barium compound supported on a Ce—Zr mixed oxide according to the Examples and Table 3 in EP 1317953. Each component was prepared separately. What we found was that when the first and second components were each prepared separately and the catalyst components were physically mixed, the NOx storage of the fresh NOx storage activity of the reconstituted catalyst was poor. However, when the separately prepared components were combined in a washcoat the NOx storage activity markedly improved.
We have discovered, very surprisingly, that a NOx absorber catalyst comprising Pt, Pd or a combination of both Pt and Pd supported on a bulk reducible oxide that is substantially free of nitrogen oxide storage material provides a beneficially active NOx absorber catalyst. In particular we have found that the NOx absorber catalysts according to the invention are particularly active: (i) for converting both desorbed NOx and NOx contained in rich exhaust gas when the NOx absorber catalyst has been aged (compared with fresh activity); and (ii) for oxidation of carbon monoxide and hydrocarbon at relatively low temperature in lean exhaust gas. Preferred embodiments of bulk reducible oxides include bulk cerium oxide (CeO2 also referred to as ceria) or bulk mixed oxide or composite oxide based on cerium oxide. An aspect of this discovery is believed to reside in that the Pt, Pd or PtPd supported on the bulk reducible oxide generates a significant exotherm and/or hydrogen gas (H2) via the water-gas shift reaction (CO+H2O→CO2+H2 (mildly exothermic)) as the exhaust gas is enriched with oxidisable components, which exotherm promotes other desirable reactions catalysed by the NOx absorber catalyst.